Various methods and apparatus have been proposed for actively reducing vibrations in a region containing a gas or liquid or in a structure of solid bodies. The concept of actively reducing vibrations consists of introducing control vibrations to combine with the vibrations in a region or structure so that the resultant vibrations in the region or structure are of a lower amplitude than the vibrations in the region or structure without the control vibrations. The active reduction of audible noise in a region has been particularly pursued, e.g., the reduction of noise in an aircraft cabin generated by a jet or propeller engine. Actively reducing vibrations is of considerable importance for low-frequency vibrations because of the difficulty in passively reducing low-frequency vibrations. Passive reduction typically refer to the use of vibration absorbing materials such as sound board in the case of noises in gases. The volume of such vibration absorbing materials needed to be effective increases considerably as the frequency of the vibration is decreased and, thus, is impractical in applications where weight and volume are constrained.
Recently, devices that reduce vibrations in a region or structure by sensing vibrations in the region or structure, decomposing the sensed vibrations into frequency components, calculating output frequency components with some frequency-domain operation, composing control vibrations from the output frequency components, and applying the control vibrations in the region or structure via actuators to reduce the sensed vibrations have been introduced. Generally referred to herein as frequency-domain vibration controllers, such controllers reduce repetitive vibrations produced by one or more repetitive vibration sources by performing a frequency-domain operation to a present cycle of the sensed vibrations to determine control vibrations and introducing the control vibrations at a later cycle of the sensed vibrations. The control vibrations reduce the sensed vibrations, which consist of the repetitive vibrations introduced by the repetitive vibration source and the control vibrations introduced by the actuators. The control vibrations can be cyclically updated to increase the amount of reduction.
U.S. Pat. No. 4,525,791 discloses a frequency-domain vibration controller for reducing repetitive vibrations in a structure that consists of an induction apparatus such as an electrical transformer or a rotating induction motor. A plurality of actuators in the form of shakers are attached to the structure, e.g., the iron core of a transformer, and the shakers apply control vibrations to the structure to reduce the vibrations in the structure. The vibration controller disclosed in the cited patent updates the control vibration of each actuator sequentially and individually with a heuristic frequency-domain operation that adjusts either the phase or amplitude of the control vibration. Since the vibration controller refines the control vibration of each actuator sequentially, it does not fully realize the control capability of the plurality of actuators that can be achieved if the control vibrations are updated simultaneously.
U.K. Patent Application No. GB2,191,063A, teaches a frequency-domain vibration controller that updates all control vibrations simultaneously. The frequency-domain controller described in this patent application is intended to be used to reduce undesired vibrations in the form of audible noise in a region such as the interior of a factory, in which, the undesired noise may be caused by repetitive machinery, for example. Loudspeakers introduce control vibrations or noises in the region to reduce the undesired noise. The plurality of control noises are cyclically updated by a frequency-domain operation involving a transfer function matrix. The transfer function matrix is updated so as to make the controller adaptive. Unfortunately, the method of updating the transfer function matrix requires several cycles and special modification of the control noises to update all elements of the transfer function matrix. Additional problems in the prior art are discussed in the following paragraphs.
Generally, in frequency-domain vibration controllers, the frequency components into which each sensed vibration is decomposed and the frequency components that compose each control vibration are the same set of frequency components; albeit each frequency component of a sensed vibration and each frequency component of a control vibration has its own amplitude and phase. On the one hand, it is desirable to use a large set of frequency components so that each sensed vibration can be accurately decomposed and so that a large number of frequency components of the sensed vibrations can be reduced with corresponding frequency components of the control vibrations. On the other hand, because the computation of the control vibrations is accomplished with an electronic processor, the number of frequency components has generally been held low so that the update cycle of decomposing the sensed vibrations into frequency components, calculating output frequency components with some frequency-domain operation, and composing control vibrations from the output frequency components is relatively fast. The present invention addresses these opposing considerations by decomposing each sensed vibration into a large number of frequency components and composing each control vibration with the same large number of frequency components while achieving a relatively fast update cycle. With each update cycle of the method of the present invention, the waveform of each control vibration approaches the optimum waveform that will maximize the reduction of the sensed vibrations.
A fast update cycle is desired so that each control vibration quickly approaches the optimum waveform that will maximize the reduction of the sensed vibrations. For each control vibration to quickly approach the optimum waveform, the update cycle must include a relatively accurate method of updating the shape of the waveform each update cycle in addition to the update cycle being relatively fast. Further, the method of updating the shape of the waveform of each control vibration should be accurate with or without changes occurring in the repetitive vibrations, the region or structure, or the frequency-domain vibration controller. With an extremely inaccurate method, a control vibration would never approach the optimum waveform regardless of the number of update cycles performed. In the opposite extreme, a perfectly accurate method would produce the optimum waveform in a single update cycle. The present invention uses an accurate and relatively fast method of updating the waveform of each control vibration. The method of updating each control vibration is accurate with or without changes occurring in a preconsidered set of parameters. The frequency of the repetitive vibrations is a parameter which changes significantly in several applications of frequency-domain vibration controllers. Therefore, frequency would likely be a preconsidered parameter, so that the method of updating each control vibration would be accurate whether or not changes occur in the frequency of the repetitive vibrations.
In some applications of frequency-domain vibration controllers, several parameters of the repetitive vibrations, the region or structure, and the frequency-domain vibration controller change significantly. In these applications, it is not practical to preconsider the parameter changes. Rather, in these applications, a method of adapting (updating) the method of updating the control vibrations is needed to maintain the accuracy of the method of updating the control vibrations. The previously mentioned foreign patent application, U.K. Patent Application No. GB2,191,063A, provides such a method of adapting. However, as was mentioned, the disclosed method of adapting requires several update cycles and requires the introduction of special control vibrations. The present invention provides an alternative method of updating each control vibration. This alternative method of updating each control vibration is completely adapted (updated) each update cycle so that the accuracy of the method of updating each control vibration is maintained or, better still, improved.